Isotopic Overprinting of Nitrification on Denitrification As a Ubiquitous and Unifying Feature of Environmental Nitrogen Cycling

Natural abundance nitrogen and oxygen isotopes of nitrate (δN and δO) provide an important tool for evaluating sources and transformations of natural and contaminant nitrate (NO) in the environment. Nevertheless, conventional interpretations of NO isotope distributions appear at odds with patterns emerging from studies of nitrifying and denitrifying bacterial cultures. To resolve this conundrum, we present results from a numerical model of NO isotope dynamics, demonstrating that deviations in δO vs. δN from a trajectory of 1 expected for denitrification are explained by isotopic over-printing from coincident NO production by nitrification and/or anammox. The analysis highlights two driving parameters: (i) the δO of ambient water and (ii) the relative flux of NO production under net denitrifying conditions, whether catalyzed aerobically or anaerobically. In agreement with existing analyses, dual isotopic trajectories >1, characteristic of marine denitrifying systems, arise predominantly under elevated rates of NO reoxidation relative to NO reduction (>50%) and in association with the elevated δO of seawater. This result specifically implicates aerobic nitrification as the dominant NO producing term in marine denitrifying systems, as stoichiometric constraints indicate anammox-based NO production cannot account for trajectories >1. In contrast, trajectories <1 comprise the majority of model solutions, with those representative of aquifer conditions requiring lower NO reoxidation fluxes (<15%) and the influence of the lower δO of freshwater. Accordingly, we suggest that widely observed δO vs. δN trends in freshwater systems (<1) must result from concurrent NO production by anammox in anoxic aquifers, a process that has been largely overlooked.